Reaction injection molding (RIM) is a process where reactive liquid components, such as a lens forming reactive polymerizable composition, are mixed, injected into a mold, and polymerized to form a plastic article. With a reactive polymerizable composition, no external curing by ultraviolet light, heat or the like is required. The article cures as a result of the chemical reaction of the liquid components in the reactive polymerizable composition. In a closed mold environment, the mold cavity components are mated prior to dosing of the reactive polymerizable composition into the mold cavity. The dosing is typically performed by a molding machine feeding the reactive polymerizable composition through a sprue, manifold or runner, and then through a gate or port for injecting the reactive polymerizable composition under pressure into the mold cavity. An outlet for run-off of excess polymerizable composition and venting for gases produced during the polymerization are typically provided within the mold cavity.
Direct injection molded ophthalmic lenses may also be made from a lens forming polymerizable composition, such as a silicone hydrogel lens forming polymerizable composition, including the silicone hydrogel lens forming polymerizable compositions disclosed in US Pub. No. 2009/0264553, entitled WETTABLE SILICONE HYDROGEL CONTACT LENSES AND RELATED COMPOSITIONS AND METHODS, filed Jul. 1, 2009, the contents of which are expressly incorporated herein by reference for all purposes. In direct injection molding using a polymerizable composition, the polymerizable composition may be injected into the mold cavity and then exposed to thermal radiation; actinic light, including UV and visible light; or other forms of radiation effective to polymerize the composition while the composition is present in the mold cavity. Alternatively or in addition thereto, the polymerizable composition can be heated in the injection unit of the injection molding machine and then injected into the mold cavity, for example to allow the polymerizable composition to flow into the mold cavity. For thermally cured polymerizable compositions, depending on the temperature of the injected polymerizable composition as it exits the barrel section of the injection molding machine, additional heating may be needed at or within the mold cavity to facilitate curing.
Direct injection molded ophthalmic lenses can also be made from a lens forming polymer composition, such as a thermoplastic or thermosetting polymeric material. In direct injection molding using a polymer composition, the polymer composition is first heated in the injection unit of the molding machine to allow the polymer material to flow or be injected into the mold cavity, and then injected into the mold cavity and cooled to solidify the composition in the shape of the mold cavity. Depending on the temperature of the injected polymer composition as it exits the barrel section of the injection molding machine, additional heating may be needed in the mold cavity to ensure the polymer composition can adequately flow into all the regions of the mold cavity.
As used herein, direct injection molding is understood to mean injection through a flow or fluid channel to deposit a lens forming composition into a lens forming cavity. The lens forming composition can comprise a lens forming reactive polymerizable composition, e.g., a lens forming reactive polymerizable composition not requiring the addition of thermal radiation or light to polymerize; a lens forming polymerizable composition, e.g., a lens forming polymerizable composition requiring the addition of radiation to polymerize; or a lens forming polymer composition, e.g., a lens forming polymer composition that is already polymerized but requires the addition of heat to be molded, such as a lens forming thermosetting polymer composition or a lens forming thermoplastic polymer composition.
With RIM using a lens forming reactive polymerizable composition, and standard injection molding using a lens forming polymerizable composition or a lens forming polymer composition, the lens forming composition is made to flow into the mold cavity through a gate, typically located on the side wall of the lens mold. Consequently, when the lens forming composition polymerizes or sets, a small amount of material can be left behind. This material left behind is referred to as a gate witness. The gate witness may be raised above the molded surface, may result in a rough area on the surface on the molded item near the location of the gate, or may both be raised above the molded surface and result in a rough area on the surface of the item. Alternatively, a gate pin may be used, which can result in a recess in the molded surface. The recess formed by the gate pin may result in the presence of a rough area on the surface on the molded item near the location of the gate pin, or may result in both a recess and a rough area on the surface of the molded item. In order to provide an ophthalmically acceptable surface, the gate witness, the recess, the rough area on the molded item, or combinations thereof, must later be treated, for example, by removing material or smoothing the cured lens, adding process steps that can be time consuming, can produce lens defects, can lower yield, and thus can increase overall production costs.